This invention relates to a drive method and drive circuit intended to compensate for degraded picture quality of moving image in a display device so designed as to display multitonal image signal making up one frame with plural subframes of different relative ratios of brightness.
The PDP (Plasma Display Panel) has recently attracted public attention as a thin, light-weight display device. Completely different from the conventional CRT driving method, the drive method of this PDP is a direct drive by digitalized image input signal. The brightness and tone emitted from the panel face depend therefore on the number of bits dealt with
The PDP may be roughly divided into AC type and DC type methods whose basic characteristics are different from each other. As for the tonal display, however, 64-tone display was the maximum reported from the trial manufacture level. The Address/Display Separation type drive method (ADS subframe method) has been proposed as an approach to solve this problem.
FIGS. 1(a) and 1(b) show the drive sequence and drive waveform of the PDP used in this ADS subframe method.
In FIG. 1(a), which gives an example of 256 tones, one frame is composed of eight subframes whose relative ratios of brightness are 1, 2, 4, 8, 16, 32, 64 and 128, respectively. Combination of this brightness of eight screens enables a display in 256 tones.
In FIG. 2(b), the respective subframes SF1 to SF8 are composed of the address duration AD1, . . . that write one screen of refreshed data and the sustaining duration ST1, . . . that defines the brightness level of these subframes. In the address duration, a wall charge is formed initially at each pixel simultaneously over all the screens, and then the sustaining pulses are given to all the screens for display. The brightness of the subframes is proportional to the number of sustaining pulses to be set to the predetermined brightness. Two hundred and fifty-six tone display is thus performed.
In such an AC drive method, the greater the number of tones, the number of bits of the address duration as preparation time for the panel to emit light and brightness within one frame duration becomes. This relatively shortens the sustaining duration as emission time, lowering thus the maximal brightness.
Hence, the brightness and tone emitted from the panel face depend on the number of bits to be dealt with. With the increased number of bits of the signal processed, the picture quality improves, but the emission brightness reduces. If, on the contrary, the number of bits of the signal processed is diminished, the emission brightness augments, but the tonal display reduces, deteriorating thus the picture quality.
The error variance processing intended to minimize the grayness error between input signal and emission brightness reducing rather the bit number of output drive signal than that of input signal is a processing to represent a pseudo-intermediate (half) tone, which is used when representing the grayness with fewer tones.
In the conventional general error variance processing circuit, the image signal of n-bit (n being 8 for instance) original pixels Ai, j enters an image signal input terminal, and passes through vertical adder and horizontal adders. Further, in the bit conversion circuit, the image signal reduces its bit number to m (4, for instance, and m less than n). After passing through the PDP drive circuit, it emits light from the PDP.
The error variance signal from said horizontal adder is compared with data stored beforehand by an error detect circuit, and the difference between this signal and the data is weighted by predetermined coefficient in an error load circuit. The error detect output is added to said vertical adder through the intermediary of the h line delay circuit that outputs the reproduction error Ej-1 produced at the pixel going back by h lines from the original pixel Aj, i, for example, by one line in the past, and at the same time, added to said horizontal adder through the intermediary of a d-dot delay circuit that outputs the reproduction error Ei-1 produced at the pixel going back by d lines from the original pixel Ai, j, for example, by one dot in the past. In general, the coefficients at said error load circuit are to be set so that their total sum may be 1 (one).
As a result, a stepwise emission brightness level represented by 4 bits is output momentarily at the output terminal of the bit conversion circuit. Nevertheless, the emission brightness levels above and below the step-like level are actually output alternately in predetermined proportion, which will be recognized as an averaged state. This allows for a correction brightness line with approximate y=x
However, the subframe lighting method was problematical in that the picture quality worsens in a part of screen when the input level of original signal somewhat changes.
In a case where 4-bit image signal scanning from SF4 to SF1 in the sequential order of brightness as shown in FIG. 2(a), the level 7 is quantized by 0111 and 8 is quantized by 1000 when the input of the first and second frames of the original signal change at levels 7 and 8, respectively. At the point of change from 7 to 8, therefore, the level becomes 01111000 as shown in FIG. 2(b) with indiscriminate emission at the levels 7 and 8. The brightness at that time reaching about 2 times the level 7 or level 8, it looks like a white line.
Conversely at the point of change from 8 to 7, the level becoming 10000111, the non-emission duration looks like a continuous black line.
The sampling signal a before conversion as shown in FIG. 3(c) and the signal b converted into the waveform of ADS subfield method as shown in FIG. 3(b) were filtered by the LPF (Low Pass Filter) with the half of frame frequency as the cutoff frequency and compared. The comparison of these signals revealed a large difference between the point of change of the image signal level from 7 to 8 and the point of change from 8 to 7 as shown in FIG. 3(e),where A represents the LPF output waveform of a, and B, that of b.
In such a display and reproduction system where the image signal is time-shared into plural subframes, there exists at a point of level change a level that does not always coincide with the change of original signal when a moving image changing in the time axial direction is displayed. This was problematical since it degrades the picture quality.
It was problematical particularly because pseudo-half tone, for example, by an error variance in one tone level was accompanied by flickering in the time axial direction.
The first purpose of this invention is to provide a method to compensate for the degradation of picture quality of a moving image arising from the half-tone display of the subframe method.
The drive method of a display device by this invention consists in that in a display unit so designed as to display a multi-tone image signal composing one frame from plural subframes of different relative ratios of brightness, two subframes of minimal brightness are arranged adjacently to each other so that the subframe selection and lighting may be possible in response to the change of image brightness in the time axial direction.
When, for example, the level of original signal changes from 7 to 8 or from 8 to 7, the brightness of 5-bit 5-screens is used, SF3, SF2, SF1 and SF1 of 4, 2, 1, and 1 are selected as the subframes for level 8, and SF3, SF2 and SF1 of 4, 2 and 1 are selected as subframes for level 7.
More materially, when one frame changes from level 7 to 8, or from 8 to 7, the level 7 is quantized at [01110] by SF3, SF2 and SF1 out of SF4, SF3, SF2, SF1 and SF1, while the level 8 is quantized at [01111] by SF3, SF2, SF1 and SF1 out of SF4, SF3, SF2, SF1 and SF1. At the point of change from level 7 to 8, the level becomes [01110] [01111], and the lighting is discontinuous at the levels 7 and 8. At the point of change from 8 to 7, the level becomes [01111] [01110] and the non-lighting is discontinuous. The brightness at these points does not therefore change greatly, which prevents the picture quality from being deteriorated.
The drive method for display device by this invention is characterized in that a correction circuit which corrects the original image signal is provided to annihilate the difference between the original image signal and emission brightness before processing the signal by the subfield drive method. The correction circuit has an M frame delay circuit which delays by M frame or frames (M being any positive integer, M=1 for example) and outputs an original image signal, a correction constant set circuit that sets, for each pixel, a correction data intended to eliminate the difference between the original image signal and emission brightness arising from the subfield drive method, based on the original image signal and M frame delay circuit, and an adder that adds the original image signal to the correction data output by the correction constant set circuit into the image signal forming the subject of the processing by the subfield drive method.
The memory (ROM for instance) in the correction constant set circuit stores beforehand the correction data intended to measure the feature representing the relationship between the original image signal and emission brightness for the display panel on which the image is displayed by the subfield drive method and to annihilate the difference between the original image signal and emission brightness as obtained for each pixel of the display panel based on the measured data. For example, data xe2x80x9c1xe2x80x9d with image signals xe2x80x9c7xe2x80x9d and xe2x80x9c8xe2x80x9d as addresses is stored as correction data when the level of the image signal changes from xe2x80x9c7xe2x80x9d to xe2x80x9c8xe2x80x9d, wherein xe2x80x9c7xe2x80x9d is the image signal (image data) going back by M frame (if M=1) or frames and xe2x80x9c8xe2x80x9d is the image signal of current frame.
Based on the image signal going back by M frame or frames that M frame delay circuit outputs (for instance, the signal of level xe2x80x9c7xe2x80x9d going back by one frame) and the image signal of current frame (for instance, signal of level xe2x80x9c8xe2x80x9d, the correction constant set circuit reads out (with the signals of level xe2x80x9c7xe2x80x9d and level xe2x80x9c8xe2x80x9d as addresses) and outputs correction data (xe2x80x9c1xe2x80x9d for example) from the incorporated memory (ROM for example). The adder adds the image signal (xe2x80x9c8xe2x80x9d for example) of current frame to the correction data output from the correction constant set circuit (xe2x80x9c1xe2x80x9d for example) and adopts the added value (xe2x80x9c9xe2x80x9d in this example) as the input image signal to the display device. We may thus eliminate the difference between the original image signal and emission brightness arising from the subfield drive method.